Fuel cell system

ABSTRACT

A fuel cell system suppresses the deterioration of an electrolyte membrane of a fuel cell. The fuel cell system comprises: a temperature rise speed calculation unit for calculating a target temperature rise speed of the fuel cell using a temperature of the fuel cell and a water content of the fuel cell; and a drive control unit for controlling a drive of the cooling water pump using the temperature rise speed of the fuel cell and the target temperature rise speed calculated by the temperature rise speed calculation unit. The drive control unit controls the drive of the cooling water pump such that a circulation amount of the cooling water is decreased when the temperature rise speed of the fuel cell is below the target temperature rise speed and controls the drive of the cooling water pump such that the circulation amount of the cooling water is increased when the temperature rise speed of the fuel cell is equal to or greater than the target temperature rise speed.

This is a 371 national phase application of PCT/JP2010/057461 filed Apr.27, 2010, claiming priority to Japanese Patent Application No.2009-126317 filed May 26, 2009, the contents of which are incorporatedherein by reference.

TECHNICAL FIELD

The present invention relates to a fuel cell system.

BACKGROUND ART

A fuel cell generates electric power through an electrochemical reactionof an oxidant gas and a fuel gas, and further generates water. Waterfreezes at or below the freezing point. Thus, when the temperatureinside the fuel cell is at or below the freezing point, the productwater freezes. Therefore, when the fuel cell is activated in cold areas,etc., it is necessary to prevent the product water from freezing insidethe fuel cell until the fuel cell is warmed up.

The below Patent Document 1 discloses a technique of, when the fuel cellis activated under a low temperature, increasing the temperature risespeed of the fuel cell by stopping a cooling water pump when thetemperature inside the fuel cell is 0° C. or below and therebypreventing the product water from freezing.

PRIOR ART REFERENCES Patent Documents

Patent Document 1: Japanese laid-open patent publication No. 2003-36874

SUMMARY OF THE INVENTION Problem to be Solved by the Invention

When the cooling water pump is stopped, the circulation of the coolingwater stops, and thus, the heat transference from the heat generatedportion in the fuel cell decreases and a power generation concentratedportion is easily formed. When the power generation concentrated portionis formed, an electrolyte membrane of the fuel cell may be deteriorated.

The present invention is for overcoming the problems by the prior artdescribed above, and its object is to provide a fuel cell system that iscapable of suppressing the deterioration of the electrolyte membrane ofthe fuel cell.

Means for Solving the Problem

In order to solve the above problem, the fuel cell system according tothe present invention comprises a fuel cell that is supplied with areactant gas and generates electric power through an electrochemicalreaction of the reactant gas; a cooling water circulation flow path forcirculating and supplying cooling water to the fuel cell; a coolingwater pump that circulates the cooling water in the cooling watercirculation flow path; and a control means for controlling, during a lowtemperature activation, a drive of the cooling water pump in accordancewith a temperature rise speed of the fuel cell.

According to the present invention, the drive of the cooling water pumpcan be controlled in accordance with the temperature rise speed of thefuel cell during a low temperature activation. Accordingly, for example,even if the fuel cell is activated at a low temperature at or below thefreezing point, the circulation amount of the cooling water can beincreased when the temperature rise speed of the fuel cell is high.Therefore, a situation where the circulation amount of the cooling wateris insufficient which causes a power generation concentrated portion tobe formed can be avoided.

In the above fuel cell system, the above control means may include: atarget temperature rise speed calculation means for calculating a targettemperature rise speed of the fuel cell using a temperature of the fuelcell and a water content of the fuel cell; and a drive control means forcontrolling a drive of the cooling water pump using the temperature risespeed of the fuel cell and the target temperature rise speed calculatedby the target temperature rise speed calculation means.

This enables the target temperature rise speed to be calculated inaccordance with the temperature and the water content of the fuel cell,and the drive of the cooling water pump to be controlled in accordancewith the target temperature rise speed. Therefore, the circulationamount of the cooling water required for preventing the deterioration ofthe electrolyte membrane included in the fuel cell can be appropriatelycontrolled.

In the above fuel cell system, the drive control means may control thedrive of the cooling water pump such that a circulation amount of thecooling water is decreased when the temperature rise speed of the fuelcell is below the target temperature rise speed and control the drive ofthe cooling water pump such that the circulation amount of the coolingwater is increased when the temperature rise speed of the fuel cell isequal to or greater than the target temperature rise speed.

This enables the circulation amount of the cooling water to becontrolled easily.

In the above fuel cell system, the control means may include a targettemperature rise speed calculation means for calculating the targettemperature rise speed of the fuel cell using a temperature of the fuelcell and a water content of the fuel cell; and a drive control means forcontrolling the drive of the cooling water pump in accordance with acirculation amount of the cooling water that is calculated using thetarget temperature rise speed calculated by the target temperature risespeed calculation means.

This enables the target temperature rise speed to be calculated inaccordance with the temperature and the water content of the fuel cell,and the drive of the cooling water pump to be controlled in accordancewith this target temperature rise speed. Therefore, the circulationamount of the cooling water required for preventing deterioration of theelectrolyte membrane included in the fuel cell can be appropriatelycontrolled.

The above fuel cell system may further comprise a target circulationamount calculation means for calculating the target circulation amountof the cooling water using the target temperature rise speed, whereinthe drive control means controls a drive amount of the cooling waterpump in accordance with the target circulation amount.

This enables the circulation amount of the cooling water to becontrolled more accurately.

In the above fuel cell system, the drive control means may stop thecooling water pump when a value obtained by subtracting the temperaturerise speed from the target temperature rise speed is equal to or greaterthan a predetermined first threshold value.

This enables the temperature of the fuel cell to be raised rapidly sincethe cooling water pump can be stopped when the difference between thetarget temperature rise speed and the temperature rise speed is equal toor greater than a first predetermined value.

In the above fuel cell system, the drive control means may drive thecooling water pump when the value obtained by subtracting thetemperature rise speed from the target temperature rise speed is equalto or smaller than a second threshold value which is a value that isequal to or smaller than the first threshold value.

This enables the inside of the fuel cell with a rising temperature to becooled and thereby suppressing the formation of a power generationconcentrated portion since the cooling water pump can be driven againwhen the difference between the target temperature rise speed and thetemperature rise speed is decreased to a second threshold value orlower.

The above fuel cell system may further comprise a temperature sensorthat is provided at the cooling water circulation flow path and measuresthe temperature of the cooling water, wherein the control meansdetermines that the fuel cell is during the low temperature activationwhen the temperature detected by the temperature sensor duringactivation of the fuel cell is equal to or smaller than a predeterminedlow temperature threshold value.

Effect of the Invention

According to the present invention, the deterioration of the electrolytemembrane of the fuel cell can be suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a configuration diagram schematically showing a fuel cellsystem in an embodiment.

FIG. 2 is a flow chart for explaining a flow of a (first) drive controlprocess of a cooling water pump.

FIG. 3 is a flow chart for explaining a flow of a (second) drive controlprocess of a cooling water pump.

FIG. 4 is a diagram showing a functional configuration of a control unitin a modification.

MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments of a fuel cell system according to the presentinvention will be described below with reference to the attacheddrawings. Each embodiment will describe an example where the fuel cellsystem according to the present invention is used as an in-vehicle powergeneration system for a fuel cell hybrid vehicle (FCHV). The fuel cellsystem according to the present invention may also be applied to variousmobile objects other than fuel cell hybrid vehicles (e.g., robots, shipsand airplanes) and also to stationary power generation systems used aspower generation facilities for constructions (houses, buildings, etc.).

First, the configuration of a fuel cell system in an embodiment will bedescribed with reference to FIG. 1. FIG. 1 is a configuration diagramschematically showing the fuel cell system in an embodiment.

As shown in FIG. 1, the fuel cell system 1 includes: a fuel cell 2 thatis supplied with an oxidant gas and a fuel gas as reaction gases andgenerates electric power through an electrochemical reaction; an oxidantgas piping system 3 that supplies air as the oxidant gas to the fuelcell 2; a fuel gas piping system 4 that supplies hydrogen as the fuelgas to the fuel cell 2; a cooling system 5 that circulates and suppliescooling water to the fuel cell 2; an electric power system 6 thatcharges and discharges an electric power of the system; and a controlunit 7 (control means) that centrally controls the entire system.

The fuel cell 2 is, for example, a polymer electrolyte fuel cell and hasa stack structure (cell stack body) in which a lot of unit cells arestacked. The unit cells each have a cathode (air electrode) on onesurface of an electrolyte constituted from an ion-exchange membrane andan anode (fuel electrode) on the other surface of the electrolyte. Theunit cell further includes a pair of separators which sandwich the anodeand the cathode therebetween. In this configuration, the fuel gas issupplied to a fuel gas flow path in one separator, while the oxidant gasis supplied to an oxidant gas flow path in the other separator, andthese reaction gases chemically react with each other to generateelectric power. The fuel cell 2 is provided with a voltage sensor V thatdetects an output voltage of the fuel cell and a current sensor A thatdetects an output current of the fuel cell 2.

The oxidant gas piping system 3 includes: a compressor 31 thatcompresses the air introduced through a filter and sends out thecompressed air as the oxidant gas; an oxidant gas supply flow path 32for supplying the oxidant gas to the fuel cell 2; and an oxidant-off gasexhaust flow path 33 for exhausting the oxidant-off gas exhausted fromthe fuel cell 2. The oxidant-off gas exhaust flow path 33 is providedwith an air back pressure valve 34 that regulates the pressure of theoxidant gas in the fuel cell 2.

The fuel gas piping system 4 includes: a fuel tank 40 serving as a fuelsupply source which stores fuel gas having a high pressure; a fuel gassupply flow path 41 for supplying the fuel gas in the fuel tank 40 tothe fuel cell 2; and a fuel circulation flow path 42 for returning afuel-off gas exhausted from the fuel cell 2 to the fuel gas supply flowpath 41. The fuel gas supply flow path 41 is provided with a pressureregulating valve 43 that regulates the pressure of the fuel gas to apreset secondary pressure, and the fuel circulation flow path 42 isprovided with a fuel pump 44 that pressurizes the fuel-off gas in thefuel circulation flow path 42 and sends the fuel gas towards the fuelgas supply flow path 41.

The cooling system 5 includes: a radiator 51 and a radiator fan 52 thatcool the cooling water; a cooling water circulation flow path 53 forcirculating and supplying the cooling water to the fuel cell 2 and theradiator 51; and a cooling water pump 54 that circulates the coolingwater in the cooling water circulation flow path 53. The outlet side ofthe fuel cell 2 of the cooling water circulation flow path 53 isprovided with a temperature sensor T1 that detects the temperature ofthe cooling water discharged from the fuel cell 2, and the inlet side ofthe fuel cell 2 of the cooling water circulation flow path 53 isprovided with a temperature sensor T2 that detects the temperature ofthe cooling water that flows into the fuel cell 2.

The electric power system 6 includes: a DC/DC converter 61; a battery 62which is a secondary battery; a traction inverter 63; a traction motor64; and various auxiliary inverters which are not shown. The DC/DCconverter 61 is a direct-current voltage converter, which has: afunction of regulating a direct-current voltage input from the battery62 and outputting the regulated voltage to the traction inverter 63; anda function of regulating a direct-current voltage input from the fuelcell 2 or the traction motor 64 and outputting the regulated voltage tothe battery 62.

The battery 62 includes stacked battery cells and provides a certainhigh voltage as a terminal voltage, the battery 62 being capable ofbeing charged with surplus power and supplying electric power in anauxiliary manner under the control of a battery computer (not shown).The traction inverter 63 converts a direct current to a three-phasealternating current, and supplies the three-phase alternating current tothe traction motor 64. The traction motor 64 is, for example, athree-phase alternating current motor, which serves as a main powersource for, for example, a fuel cell hybrid vehicle equipped with thefuel cell system 1. The auxiliary inverters are electric motor controlunits which control the drive of respective motors, and the auxiliaryinverters convert a direct current to a three-phase alternating currentand supply the three-phase alternating current to the respective motors.

The control unit 7 detects the amount of operation of an accelerationmember (an accelerator) provided in a fuel cell hybrid vehicle, receivescontrol information such as an acceleration request value (e.g., theamount of power generation required by power-consuming apparatuses suchas the traction motor 64), and controls the operation of variousappliances in the system. Examples of the power-consuming apparatusesmay include, in addition to the traction motor 64, auxiliary apparatusesrequired for operating the fuel cell 2 (e.g., motors for the compressor31, fuel pump 44, cooling water pump 54 and radiator fan 52, etc.);actuators used in various apparatuses relevant to the travel of thevehicle (e.g., a speed change gear, a wheel control apparatus, asteering gear and a suspension); and an air-conditioning apparatus (airconditioner), lighting equipment, audio system, etc. which are providedin a passenger compartment.

The control unit 7 physically includes, for example: a CPU; a memory 70;and an input-output interface. The memory 70 includes, for example, aROM that stores a control program and control data which are processedby the CPU and a RAM primarily used as various work areas for controlprocessing. These elements are connected to each other via a bus. Theinput-output interface is connected to various sensors such as thetemperature sensors T1 and T2, as well as various drivers, for example,for driving the cooling water pump 54, etc.

The CPU receives detection results in the respective sensors via theinput-output interfaces, and processes the received detection resultsusing various types of data in the RAM, in accordance with the controlprogram stored in the ROM, thereby performing the control process of thecooling water pump which will be described later. The CPU outputscontrol signals to the various drivers via the input-output interfaces,thereby controlling the entire fuel cell system 1.

The control unit 7 functionally includes: a temperature rise speedcalculation unit 71 (target temperature rise speed calculation means);and a drive control unit 72 (drive control means). The memory 70 of thecontrol unit 7 stores various maps referenced by the temperature risespeed calculation unit 71 and the drive control unit 72.

The temperature rise speed calculation unit 71 calculates thetemperature rise speed and the target temperature rise speed of the fuelcell 2. The temperature rise speed, for example, can be calculated usingthe temperature of the fuel cell 2. More specifically, the temperaturerise speed can be calculated by obtaining values of the temperaturesensor T1 for every predetermined interval and calculating the degree ofchange.

The target temperature rise speed, for example, can be calculated usingthe water content of the fuel cell and the temperature of the fuel cell.More specifically, the target temperature rise speed, for example, canbe calculated by referencing the maps that associate the water contentof the fuel cell with the target pressure rise speed of the fuel cellfor each temperature of the fuel cell and stores the result.

The target temperature rise speed that is stored in the map is set to belower as the water content of the fuel cell decreases and higher as thewater content of the fuel cell increases and is set to be lower as thetemperature of the fuel cell increases and higher as the temperature ofthe fuel cell decreases. That is, since the possibility of the productwater freezing is low when the water content of the fuel cell is smalland the temperature of the fuel cell is high, the target temperaturerise speed is decreased to improve the operational efficiency. On theother hand, since the possibility of the product water freezing is highwhen the water content of the fuel cell is high and the temperature ofthe fuel cell is low, the target temperature rise speed is increased torapidly raise the temperature.

For example, a value of a temperature sensor T1 can be used as thetemperature of the fuel cell that is used when calculating the targettemperature rise speed. The water content of the fuel cell that is usedwhen calculating the target temperature rise speed, for example, can becalculated using the integrated value of the output current of the fuelcell 2. More specifically, the following formula 1 is used to calculatethe target temperature rise speed.[Formula 1]h=h ₀ +ΣI _(FC) ×a  Formula 1

The h in the above formula 1 is the water content of the fuel cell, h₀is the initial value of the water content, I_(FC) is the output currentof the fuel cell, and a is the conversion factor.

The initial value h₀ of the above formula 1 may use, when starting thefuel cell 2, the impedance of the fuel cell 2 that was measured when thefuel cell 2 was terminated last. On the other hand, when restarting thenormal operation after intermittent operation of the fuel cell, thewater balance in the fuel cell 2 that was calculated during the previousnormal operation may be used as the initial value h₀ of the aboveformula 1. The water balance in the fuel cell 2 can be calculated bysubtracting the water content that is carried away as water vapor by theoxidant gas supplied to the fuel cell 2 from the water content generatedby power generation of the fuel cell 2.

By calculating the target temperature rise speed using the water contentof the fuel cell 2 and the temperature of the fuel cell 2, the targettemperature rise speed can be determined in accordance with the state ofthe fuel cell that is affected by the temperature and the water contentof the fuel cell 2. Thus, it is possible to appropriately control thecirculation amount of the cooling water required for preventingdeterioration of the electrolyte membrane included in the fuel cell 2.

The drive control unit 72 controls the drive of the cooling water pump54 in accordance with the target temperature rise speed and thetemperature rise speed calculated by the temperature rise speedcalculation unit 71. More specifically, when the temperature rise speedis below the target temperature rise speed, the drive control unit 72controls the drive of the cooling water pump 54 such that thecirculation amount of the cooling water is decreased, and when thetemperature rise speed is equal to or greater than the targettemperature rise speed, the drive of the cooling water pump 54 iscontrolled such that the circulation amount of the cooling water isincreased.

By controlling the drive of the cooling water pump 54 in this manner,for example, when the water content of the fuel cell is small and thetemperature of the fuel cell is high, the target temperature rise speedcan be kept low by the above temperature rise speed calculation unit 71.As a result, the temperature rise speed exceeds the target temperaturerise speed, and it is possible to increase the circulation amount of thecooling water. This enables the formation of a power generationconcentrated portion that easily occurs when the water content of thefuel cell is small and the temperature of the fuel cell is high to besuppressed. On the other hand, when the water content of the fuel cellis large and the temperature of the fuel cell is low, the targettemperature rise speed can be greatly increased by the temperature risespeed calculation unit 71. As a result, the temperature rise speed fallsbelow the target temperature rise speed, and it is possible to decreasethe circulation amount of the cooling water and prioritize thetemperature rise. This enables freezing of the product water that easilyoccurs when the water content of the fuel cell is high and thetemperature of the fuel cell is low to be suppressed.

When the value obtained by subtracting the temperature rise speed fromthe target temperature rise speed is equal to or greater than a firstthreshold value, the drive control unit 72 stops the cooling water pump54. The first threshold value is a determination value for determiningwhether or not to stop the cooling water pump in order to accelerate thetemperature rise of the fuel cell 2. Accordingly, for example, whentaking into consideration the difference between the target temperaturerise speed and the temperature rise speed, the first threshold value maybe set as a limit value for determining that it is more effective tostop the cooling water pump 54 and prioritize the temperature rise ofthe fuel cell rather than driving the cooling water 54. This enables arapid rise in temperature of the fuel cell 2 since the cooling waterpump 54 can be stopped when the difference between the targettemperature rise speed and the temperature rise speed is equal to orgreater than the first threshold value.

When the value obtained by subtracting the temperature rise speed fromthe target temperature rise speed is equal to or smaller than a secondthreshold value, the drive control unit 72 drives the cooling water pump54. The second threshold value may be set as a value equal to or smallerthan the first threshold value. This enables the inside of the fuel cell2 with an increasing temperature to be cooled and the formation of apower generation concentrated portion to be suppressed since the coolingwater pump 54 can be driven again when the difference between the targettemperature rise speed and the temperature rise speed is decreased tothe second threshold value or lower.

It is preferable that the second threshold value is set as a valuesmaller than the first threshold value. This is because, if the secondthreshold value is set as the same value as the first threshold value,the cooling water pump 54 would frequently repeat the stop/drive of thecooling water pump 54 when the temperature rise speed varies near thethreshold value. Therefore, setting the second threshold value to avalue smaller than the first threshold value can prevent a frequentrepetition of a control for stopping and driving the cooling water pump54.

Next, a (first) drive control process of a cooling water pump that isperformed in a fuel cell system in an embodiment will be described belowwith reference to the flow chart shown in FIG. 2. This drive controlprocess is a process repeatedly performed during an activation processthat is performed when the fuel cell is activated.

First, the control unit 7 determines whether or not the fuel cell isduring a low temperature activation (Step S101). When this determinationis NO (Step S101; NO), the control unit 7 ends the present drive controlprocess. The determination as to whether or not it is during a lowtemperature activation can be determined, for example, as describedbelow. It is determined that the fuel cell is during a low temperatureactivation when the temperature detected by the temperature sensor T1during the activation of the fuel cell 2 is equal to or smaller than apredetermined low temperature threshold value. For example, 0° C. whichis a freezing point may be used as the predetermined low temperaturethreshold value.

On the other hand, when it is determined that the fuel cell is during alow temperature activation in the determination in the above Step S101(Step S101; YES), the drive control unit 72 determines whether or notthe temperature rise speed is below the target temperature rise speed(Step S102).

When it is determined that the temperature rise speed is below thetarget temperature rise speed in this determination (Step S102; YES),the drive control unit 72 controls the drive of the cooling water pump54 such that the circulation amount of the cooling water is decreased(Step S103).

On the other hand, when it is determined that the temperature rise speedis equal to or greater than the target temperature rise speed in thedetermination of the above Step S102 (Step S102; NO), the drive controlunit 72 controls the drive of the cooling water pump 54 such that thecirculation amount of the cooling water is increased (Step S104).

A (second) drive control process of a cooling water pump that isperformed in a fuel cell system in an embodiment will be described belowwith reference to the flow chart shown in FIG. 3. This (second) drivecontrol process is performed in parallel with the above (first) drivecontrol process.

First, the control unit 7 determines whether or not the fuel cell isduring a low temperature activation (Step S201). When this determinationis NO (Step S201; NO), the control unit 7 ends the present drive controlprocess.

On the other hand, when it is determined that the fuel cell is during alow temperature activation in the determination in the above Step S201(Step S201; YES), the drive control unit 72 determines whether or notthe value obtained by subtracting the temperature rise speed from thetarget temperature rise speed calculated by the temperature rise speedcalculation unit 71 is equal to or greater than the first predeterminedvalue (Step S202). When this determination is NO (Step S202; NO), thecontrol unit 7 proceeds to a process in Step S204 which will bedescribed later.

On the other hand, when it is determined that the value obtained bysubtracting the temperature rise speed from the target temperature risespeed is equal to or greater than the first threshold value in thedetermination of the above Step S202 (Step S202; YES), the drive controlunit 72 stops the cooling water pump 54 (Step S203).

Subsequently, the drive control unit 72 determines whether or not thevalue obtained by subtracting the temperature rise speed from the targettemperature rise speed calculated by the temperature rise speedcalculation unit 71 is equal to or smaller than the second thresholdvalue (Step S204). When this determination is NO (Step S204; NO), thecontrol unit 7 ends the present drive control process.

On the other hand, when it is determined that the value obtained bysubtracting temperature rise speed from the target temperature risespeed is equal to or smaller than the second threshold value in thedetermination of the above Step S204 (Step S204; YES), the drive controlunit 72 drives the cooling water pump 54 (Step S205).

As described above, according to the fuel cell system 1 in anembodiment, since the drive of the cooling water pump 54 can becontrolled in accordance with the temperature rise speed of the fuelcell 2 during a low temperature activation, even if the fuel cell isactivated at a low temperature at or below the freezing point, thecirculation amount of the cooling water can be increased when thetemperature rise speed of the fuel cell 2 is high. Therefore, asituation where the circulation amount of the cooling water isinsufficient which causes a power generation concentrated portion to beformed can be avoided, thereby enabling a suppression of thedeterioration of an electrolyte membrane of the fuel cell 2.

Modification

The drive control unit 72 in the above embodiment controls the drive ofthe cooling water pump 54 in accordance with the target temperature risespeed and the temperature rise speed, but the method for controlling thedrive of the cooling water pump 54 is not limited to such. For example,the target circulation amount of the cooling water may be calculatedusing the target temperature rise speed to control the drive of thecooling water pump in accordance with this target circulation amount.

A fuel cell system in the present modification will be described below.The fuel cell system in the present modification is different from thefuel cell system in the embodiment described above in that this fuelcell system is further provided, in addition to the functions of thecontrol unit 7 in the embodiment described above, with a circulationamount calculation unit 73 which will be described later. Since theother configurations are the same as the respective configurations inthe embodiment, the difference between the embodiment and themodification will be mainly described below.

The control unit 7 in the present modification as shown in FIG. 4includes the above temperature rise speed calculation unit 71 and drivecontrol unit 72, and a circulation amount calculation unit 73 (targetcirculation amount calculation means).

The circulation calculation unit 73 calculates the target circulationamount of the cooling water using the target temperature rise speedcalculated by the temperature rise speed calculation unit 71. Thecirculation amount calculation unit 73 can calculate the targetcirculation amount of the cooling water using, for example, the belowformulas 2 to 4. More specifically, the target circulation amount of thecooling water can be calculated by substituting the below formulas 3 and4 into the below formula 2 and determining the circulation amount Y ofthe cooling water, and then setting the target circulation amount of thecooling water to be equal to or smaller than the circulation amount Y ofthe cooling water.

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack & \; \\{\frac{S}{C_{FC}} = {Q_{FC} - Q_{OUT}}} & {{Formula}\mspace{14mu} 2}\end{matrix}$

The S in the above formula 2 is the target temperature rise speed[K/sec], C_(FC) is the heat capacity [kJ/K] of the fuel cell, Q_(FC) isthe amount of heat generation [kW] of the fuel cell, and Q_(out) is theamount of discharge [kW] of the fuel cell.[Formula 3]Q _(FC)=[(V _(T) ×n)−V _(FC) ]×I _(FC)÷1000  Formula 3

The Q_(FC) of the above formula 3 is the amount of heat generation [kW]of the fuel cell, V_(T) is the theoretical electromotive voltage [V]when generating heat in a unit cell, n is the number of unit cellsincluded in the fuel cell, V_(FC) is the output voltage [V] of the fuelcell, and I_(FC) is the output current of the fuel cell. The amount ofheat generation Q_(FC) of the fuel cell can be determined using thecurrent temperature of the fuel cell and the current water content ofthe fuel cell. Therefore, the correlation thereof may be derived inadvance by experiments, etc. and stored in a map, and then the amount ofheat generation Q_(FC) of the fuel cell may be calculated with referenceto this map.[Formula 4]Q _(OUT) =k×(T _(IN) −T _(OUT))×Y  Formula 4

The Q_(out) of the below formula 4 is the amount of discharge [kW] ofthe fuel cell, k is the specific heat [kJ/kgK] of the cooling water,T_(IN) is the cooling water temperature at the fuel cell inlet side,T_(OUT) is the cooling water temperature at the fuel cell outlet side,and Y is the flow rate [kg/sec] of the cooling water.

The drive control unit 72 in the present modification controls the driveof the cooling water pump 54 in accordance with the target circulationamount of the cooling water calculated by the circulation amountcalculation unit 73. That is, the drive control unit 72 controls thedrive of the cooling water pump 54 in accordance with the targetcirculation amount of the cooling water calculated using the targettemperature rise speed S. This enables a more accurate control of thecirculation amount of the cooling water.

INDUSTRIAL APPLICABILITY

The fuel cell system according to the present invention is suitable forsuppressing the deterioration of an electrolyte membrane of a fuel cell.

DESCRIPTION OF REFERENCE NUMERALS

1 . . . Fuel cell system; 2 . . . Fuel cell; 3 . . . Oxidant gas pipingsystem; 4 . . . Fuel gas piping system; 5 . . . Cooling system; 6 . . .Electric power system; 7 . . . Control unit; 53 . . . Cooling watercirculation flow path; 54 . . . Cooling water pump; 70 . . . Memory; 71. . . Temperature rise speed calculation unit; 72 . . . Drive controlunit; 73 . . . Circulation amount calculation unit; T1 . . . Temperaturesensor; T2 . . . Temperature sensor; V . . . Voltage sensor; A . . .Current sensor.

What is claimed is:
 1. A fuel cell system comprising: a fuel cell thatis supplied with a reactant gas and generates electric power through anelectrochemical reaction of the reactant gas; a cooling watercirculation flow path for circulating and supplying cooling water to thefuel cell; a cooling water pump for circulating the cooling water in thecooling water circulation flow path; a target temperature rise speedcalculation unit programmed to calculate a target temperature rise speedof the fuel cell using a temperature of the fuel cell and a watercontent of the fuel cell, the target temperature rise speed of the fuelcell being lower as the temperature of the fuel cell increases and thewater content of the fuel cell decreases and the target temperature risespeed of the fuel cell being higher as the temperature of the fuel celldecreases and the water content of the fuel cell increases; and a drivecontrol unit programmed to control, during an activation below afreezing point, a drive of the cooling water pump such that acirculation amount of the cooling water is decreased when thetemperature rise speed of the fuel cell is below the target temperaturerise speed and such that the circulation amount of the cooling water isincreased when the temperature rise speed of the fuel cell is equal toor greater than the target temperature rise speed.
 2. The fuel cellsystem according to claim 1, wherein the drive control unit isprogrammed to stop the cooling water pump when a value obtained bysubtracting the temperature rise speed of the fuel cell from the targettemperature rise speed is equal to or greater than a predetermined firstthreshold value.
 3. The fuel cell system according to claim 2, whereinthe drive control unit is programmed to drive the cooling water pumpwhen the value obtained by subtracting the temperature rise speed of thefuel cell from the target temperature rise speed is equal to or smallerthan a second threshold value which is a value that is equal to orsmaller than the first threshold value.
 4. The fuel cell systemaccording to claim 1, further comprising a temperature sensor that isprovided at the cooling water circulation flow path and measures atemperature of the cooling water, wherein the drive control unitdetermines whether or not the fuel cell is during an activation below afreezing point based on the temperature detected by the temperaturesensor during an activation of the fuel cell.